Inspection of semiconductor wafers (and other objects, such as reticles, flat-panel displays, photomasks, and the like) often involves scanning the entire wafer or other object. Generally speaking, there are three main types of scanning, which are shown in FIG. 11. For instance, in point scanning, the instantaneous field of view comprises a single point and the scanning is carried out by applying a saw-like relative movement between the wafer and the illumination/detection point. In line scanning, the instantaneous field of view comprises a narrow line of pixels. Wafer scanning is carried out by applying a raster-like relative movement between the wafer and the detector. In area scanning, the instantaneous field of view comprises a two-dimensional area and the scanning is carried out by applying a discrete raster-like relative movement between the wafer and the detector.
Optical wafer inspection can require a relatively long period of time, especially when high magnification is used. Presently, throughput of only a few wafers per hour is common. Where throughput needs to be increased, the inspection operator faces a limited number of choices. For example, lower magnification can be used so that a larger area is included in the instantaneous field of view. However, due to the lower magnification, smaller defects may be missed.
Another option is to scan only a part of the wafer. Examples of partial scanning include scanning some, but not all of a wafer based on a (manual and/or automatic) designation. For instance, FIGS. 12A-12B show scanning of some dies, but not other dies. FIG. 12A shows an inspection in which alternating rows of dies 12 in a wafer 10 are scanned. Other partial scans are possible. FIG. 12B shows an inspection in which the top half, but not the bottom half, of a wafer 10 is scanned. FIG. 12C shows an inspection in which part of every die is scanned, namely an inspection where every other slice of a wafer 10 is scanned.
However, the use of partial scanning runs the risk that an area comprising a defect may be missed entirely if the area is included in a part of the wafer that is not scanned.